The present invention relates to sensor for detecting magnetic anomalies, and more specifically, magnetic sensors which take advantage of the property of giant magnetoresistance.
Magnetic sensors are used in a wide variety of applications for detecting magnetic anomalies. An example of a common magnetic sensor is a fluxgate magnetometer. A conventional fluxgate magnetometer 100 wound on a ferrite torus core 102 is shown in FIG. 1. Magnetometer 100 has a drive coil 104 which is driven with a sine wave of frequency f. The function of the drive coil is to alternately saturate the ferrite torus core in the clockwise and counterclockwise directions. A sense coil 106 is also included which senses any net flux in the horizontal direction, i.e., the direction normal to the plane of the sense coil, and only during the time in the drive cycle when the drive current is nearly zero. That is, the horizontal net flux only exists when the current in drive coil 104 is near zero, i.e., at the zero crossings of the sine wave input. Because this occurs twice in each period of the input signal, the frequency of the output signal on sense coil 106 is twice that of the input signal, i.e., 2f, thus reducing noise on the sensor""s output.
FIGS. 2(a)-2(d) illustrate the changing magnetic configuration of magnetometer 100 during one period of the input signal. The direction of magnetization is indicated by the arrows superimposed in ferrite core 102. In FIG. 2(a), the input drive current is maximum and positive resulting in saturation of core 102 in the counterclockwise direction. When the drive current is reduced to zero, the magnetization of core 102 responds to the external field as shown in FIG. 2(b). When the drive current is maximum and negative, core 102 is saturated in the clockwise direction as shown in FIG. 2(c). Finally, when the input drive current again reaches zero, the magnetization again responds to the external field (FIG. 2(d)).
A common application for fluxgate magnetometers is a conventional fluxgate gradiometer as shown in FIG. 3. Gradiometer 300 employs two fluxgate magnetometers 302 and 304 in two different positions and having the same input drive current in drive coil 306. The sense coils 308 and 310 of the magnetometers are connected to a differential amplifier 312, the output of which represents the difference between the magnetic fields at the two magnetometer locations.
Unfortunately, conventional fluxgate magnetometers and the devices of which they are part (e.g., fluxgate gradiometers) suffer from some serious drawbacks. First, these devices are too large and power hungry to be used in microsensing applications. In addition, they can be prohibitively expensive for many applications. Finally, such devices have not heretofore been fabricated using integrated circuit techniques. It is therefore desirable to provide magnetic sensing technology which is inexpensive, suitable for microsensing applications, and amenable to integrated circuit fabrication techniques.
According to the present invention, magnetic sensing technology is provided operation of which is based on the property of multi-layer magnetic thin film structures known as giant magnetoresistance (GMR). According to a specific embodiment, a magnetic sensor is provided which is based on a GMR device referred to herein as a xe2x80x9ctranspinnor.xe2x80x9d A transpinnor is a multi-functional, active solid-state device comprising a network of GMR thin film elements which has characteristics similar to both transistors and transformers. Like a transistor, the transpinnor can be used for power amplification, current amplification, voltage amplification, or logic. Like a transformer, the transpinnor can be used to step voltages and currents up or down with the input resistively isolated from the output.
According to a specific embodiment of the present invention, a transpinnor-based magnetometer is provided having four resistive elements exhibiting GMR in a bridge configuration. Two input conductors are each inductively coupled to two of the resistive elements with a sine-wave drive current applied to one and a bias current applied to the other. When the resistance of the two arms of the bridge are equal, the bridge is balanced and there is no output current. When the field imposed by a magnetic anomaly causes the resistances to become unequal, the bridge is unbalanced and produces an output representative of the external magnetic field.
Thus, the present invention provides a device for sensing a magnetic anomaly which includes a network of thin film elements exhibiting giant magnetoresistance. A first conductor is inductively coupled to a first subset of the thin film elements for supplying a drive current to the device. A second conductor is inductively coupled to a second subset of the thin film elements for providing a bias current to the device. The network of thin film elements generates an output signal in response to an external magnetic field oriented in a first direction relative to the applied drive current, the external magnetic field being representative of the magnetic anomaly.
According to another specific embodiment, a gradiometer is provided also comprising a network of thin film elements exhibiting giant magnetoresistance. A first conductor is inductively coupled to a first subset of the thin film elements for supplying a drive current to the device. A second conductor is inductively coupled to a second subset of the thin film elements for providing a bias current to the device. The network of thin film elements generates an output signal in response to an external magnetic field, the external magnetic field being representative of a magnetic anomaly. The output signal is representative of one component of a gradient tensor associated with the external magnetic field.
According to still further embodiments, a plurality of such gradiometers are configured to detect multiple components of the external field""s gradient tensor. According to some of these embodiments, a magnetometer designed according to the invention is also included with the gradiometers, the magnitude data from the magnetometer and spatial derivative data from the multiple gradiometers being combined to determine the size, distance, and direction of travel of the magnetic anomaly.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.